Quantum impurity with 2/3 local moment in 1D quantum wires: an NRG study

Abstract: We study a Kondo state that is strongly influenced by its proximity to an w^-1/2 singularity in the metallic host density of states. This singularity occurs at the bottom of the band of a 1D chain, for example. We first analyze the non-interacting system: A resonant state e_d, located close to the band singularity, suffers a strong `renormalization', such that a bound state is created below the bottom of the band in addition to a resonance in the continuum. When e_d is positioned right at the singularity, the spectral weight of the bound state is 2/3, irrespective of its coupling to the conduction electrons. The interacting system is modeled using the Single Impurity Anderson Model, which is then solved using the Numerical Renormalization Group method. We observe that the Hubbard interaction causes the bound state to suffer a series of transformations, including level splitting, transfer of spectral weight, appearance of a spectral discontinuity, changes in binding energy (the lowest state moves farther away from the bottom of the band), and development of a finite width. When e_d is away from the singularity and in the intermediate valence regime, the impurity occupancy is lower. As e_d moves closer to the singularity, the system partially recovers Kondo regime properties, i.e., higher occupancy and lower Kondo temperature T_K. The impurity thermodynamic properties show that the local moment fixed point is also strongly affected by the existence of the bound state. When e_d is close to the singularity, the local moment fixed point becomes impervious to charge fluctuations (caused by bringing e_d close to the Fermi energy), in contrast to the local moment suppression that occurs when e_d is away from the singularity. We also discuss an experimental implementation that shows similar results to the quantum wire, if the impurity's metallic host is an armchair graphene nanoribbon.